Worker response to exercise in the heat while wearing lightweight, disposal Tyvek coveralls of the type worn during asbestos (1332214) removal operations in combination with four different levels of respiratory protection was investigated. The subjects included nine men, nonsmokers, aged 22 to 34 years. They were experienced fire fighters or emergency service personnel with prior experience using respiratory and protective clothing. The men participated in treadmill walking at a set speed of 4 kilometers per hour (kph) at 0 percent elevation in a controlled environment of 33.9 degrees-C and 50 percent relative humidity. The tests continued for 120 minutes with repeated work/rest intervals of 26 minutes work and 4 minutes rest. The four conditions included: one, control (a lightweight low resistance mask); two, an air purifying, full facepiece respirator with dual high efficiency filters, (HEPA); three, a supplied air, pressure demand respirator with escape filter (SAR); and four, an open circuit, pressure demand, self contained breathing apparatus, (SCBA). Heart rate, and skin and rectal temperatures were continuously recorded during the test periods. Subjective data were also collected. The heart rate varied by less than 8 beats per minute between the control conditions and the heaviest condition, number four. The skin and rectal temperature likewise varied only slightly, 0.2 degree for skin and 0.1 degree for rectal temperatures. The only subjective changes were that the respirator load was significantly different between ensembles. The authors conclude that more protective respiratory equipment may be associated with only minimal additional physiological and subjective stress in selected low work load asbestos abatement industry work settings.